In optical communications of digital signals, an information carrier is not an electrical pulse but a light pulse. A light-emitting diode and a semiconductor laser are generally popular as its light sources. The semiconductor laser has an advantage in facilitation of high-speed driving, but is thermally unstable. In order to stabilize the operation of the semiconductor laser, a circuit is undesirably complicated, resulting in an economical disadvantage.
To the contrary, when a light-emitting diode is used as a light source, optical communication can be performed using a simple drive circuit. In particular, in optical communication using an optical fiber, a red light-emitting diode for emitting red light having a wavelength of about 660 nm in a visible range is excellent in transmission loss characteristics of an optical fiber for this wavelength. In addition, the red light-emitting diode is relatively easily accessible and is very popular.
A basic block diagram of a conventional light-emitting diode drive circuit for driving a light-emitting diode upon reception of an electrical pulse signal is shown in FIG. 6. The circuit shown in FIG. 6 comprises a signal input circuit 61 for outputting drive pulse signals S2 and S3 in response to an input pulse signal S1 and a current supply circuit 63 for supplying a drive current to a light-emitting diode 62.
Conventional examples of the main part (current supply circuit 63) of the light-emitting diode drive circuit are shown in FIGS. 7(a), 7(b), and 8.
The conventional examples shown in FIGS. 7(a) and 7(b) are circuits each arranged such that a transistor 71 which receives an input signal Si at its base and a parallel circuit of a capacitor Cs and a resistor Rs are connected in series with a light-emitting diode 62.
The conventional example shown in FIG. 8 comprises two transistors 80 and 81 which have identical characteristics and emitters of which are connected to each other. The collector of one transistor 81 is connected to a voltage source Vcc through a light-emitting diode 62, and the collector of the other transistor 81 is directly connected to the voltage source Vcc. The commonly connected emitters of the transistors 80 and 81 are grounded through a collector-emitter path of a transistor 86 and a resistor 85. Note that the transistor 86 constitutes a constant current circuit 87 together with a transistor 82, and resistors 83 and 84 and the resister 85.
Drive pulse signals S2 and S3 output from a signal input circuit 61 are input to the bases of the differentially connected transistors 80 and 81, respectively. Since the drive pulse signals S2 and S3 have an inverse relationship, one transistor 80 is kept ON, while the other transistor 81 is kept OFF. To the contrary, when one transistor 80 is kept OFF, the other transistor 81 is kept ON. By controlling operations of the differentially connected transistors 80 and 81 as described above, the light-emitting diode 62 is driven to be turned on/off.
In optical communication which requires high-speed response, however, since the red light-emitting diode having a short wavelength range requires a longer charge/discharge time at the time of turn-ON/OFF operation of the light-emitting diode than that of a general infrared light-emitting diode. Therefore, waveform distortion of a light pulse output is increased, and a response speed is decreased, thus posing problems.
In a light-emitting diode drive circuit to drive a light-emitting diode having these problems in response to an input signal, only a light output pulse having response characteristics depending on a low response speed unique to the light-emitting diode can be obtained in accordance with a system for simply turning on/off a current supplied to the light-emitting diode.
In addition, a semiconductor light-emitting element such as a light-emitting diode has a parallel electrostatic capacitance (to be simply referred to as a capacitance) between anode and cathode terminals of the semiconductor light-emitting element. For this reason, when a light-emitting diode is to be driven at high speed, charge/discharge time of the capacitance equivalently present in each light-emitting diode cannot be neglected. A trailing phenomenon occurs in leading and trailing edge portions of an output waveform due to light pulse output waveform distortion.
Various conventional proposals have been made to prevent a phenomenon in which leading and trailing periods of the light pulse output are prolonged. These proposals are exemplified by a light-emitting diode drive circuit (Japanese Patent Laid-Open No. 56-87189) for separating a route for charging a capacitance of a light-emitting diode from a route for discharging the capacitance of the light-emitting diode, a light-emitting diode drive circuit (Japanese Patent Laid-Open No. 58-137340) for adding a peaking current to a drive current during a rise time to shorten the rise time, and for reverse-biasing the light-emitting diode during a fall time, a light-emitting diode drive circuit (Japanese Patent Laid-Open No. 60-180232) in which an equivalent voltage source and a resistor are connected in parallel with a light-emitting diode, a light-emitting diode drive circuit (Japanese Utility Model Laid-Open No. 62-167436) in which a transistor is connected in parallel with a light-emitting diode to short-circuit both terminals of the light-emitting diode during a fall time, a light-emitting diode drive circuit (Japanese Patent Laid-Open No. 63-234568) in which a transistor is connected in parallel with a light-emitting diode to short-circuit both terminals of the light-emitting diode during fall time, and turn-ON/OFF switching is performed by a differential circuit, and a light-emitting diode drive circuit (Japanese Patent Laid-Open No. 64-64267) in which a transistor is connected in parallel with a light-emitting diode, a constant current source is connected in series with the light-emitting diode, and a differentiation circuit is arranged between the input terminal of the transistor and a connecting point between the light-emitting diode and the constant current source.
In each of the conventional light-emitting diode drive circuits, however, good response characteristics are not necessarily obtained for high-speed driving of a light-emitting diode (particularly, a red light-emitting diode having a short wavelength range).
In each of the conventional light-emitting diode drive circuits, a current consumed by the light-emitting diode varies to generate noise on a power source line. This noise causes a circuit operation error, and reliability of the circuit is degraded. In particular, when a high-speed operation is to be performed, this noise is much produced.